1. Field of the Invention
The present invention relates to a band-pass filter
2. Description of the related art
Recently, a strong demand has arisen for employing a monolithic IC arrangement in various filters A circuit in which a monolithic IC arrangement can be employed must have low element sensitivity and a simple circuit arrangement as possible.
An example of filters designed to satisfy the above requirements is a leap frog type band-pass filter by Paul R. Gray et al. This filter is described in IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL SC-19, No. 6, DECEMBER 1984, "High-Frequency CMOS Continuous-Time Filters" (PP. 939-949).
This circuit is based on a ladder type LC filter having low element sensitivity, i.e., not easily adversely affected by variations in element characteristics. Therefore, since relative accuracy between elements can be increased if an IC arrangement is employed in a circuit constituted by elements of the same size, scaling can be performed in a relationship between L and C so that time constants of all integrating circuits can be equalized. In addition, according to this method, a filter can be advantageously constituted by only capacitive-coupling of the integrating circuit loops without using any adders and any buffer circuits.
However, the characteristics of the above circuit are not completely theoretical. This is because an operation without strictness such as narrow band approximation is performed, regardless of the above merits. For this reason, pass band characteristics are disturbed, and especially in a wide band filter, desired characteristics cannot be obtained, thereby posing a serious problem. This is because modifications including the narrow band approximation in which transfer characteristics do not strictly coincide with those of the original LC filter are performed at several portions.